Marketing research techniques have been developed in which a substitute television signal in a substitute channel, containing a commercial the effectiveness of which is to be assessed, is substituted for a normal television signal in a normal channel in homes of selected test viewers so that the effectiveness of the commercial can be evaluated. This allows the promoter of a service or product to assess the reaction of a small, demographically controlled panel of test viewers before the wide airing of a commercial which may prove ineffective.
One example of such a television signal substitution system is disclosed in U.S. Pat. No. 4,404,589. As there disclosed, substitute television program signals are transmitted in at least one substitute channel along with signal substitution control signals. A control box or terminal at each test viewer receiver responds to the signal substitution control signals by selectively switching to a substitute television program from a normal program. The signal substitution control signals include a number of different terminal command signals and a number of different event command signals. Each of the terminal command signals includes a respective test viewer address signal for identifying a respective test viewer receiver and a number of event identification signals identifying respective signal substitution events in which this terminal is to participate. Each of the event command signals includes a respective event address signal corresponding to a respective event, an appropriate substitution control command, a substitute channel identification signal, and one or more normal channel identification signals for identifying the normal channels from which the receiver is to be switched. The current event command signals corresponding to each allowable event address are stored in the terminal for later correlation with the terminal's participation event list and to the viewer's selected channel signal. When the viewer selected channel corresponds to a normal channel identification signal associated with a current event command whose event address signal corresponds to an event in which the respective terminal is to participate, the substitute channel is substituted for the channel selected by the viewer for a period determined by the event command signals. Subsequent responses to the events, such as purchases of the respective viewers, are then individually tabulated and analyzed against the responses of viewers receiving the normal signals.
When a viewer changes channels on a modern television receiver, the channel change is carried out in, for example, about a quarter of a second. The change is accompanied by momentary disruption of the picture and a sound pop or a period of sound muting. When a market research company causes a channel substitution, it is desirable that the substitution be carried out so quickly and unobtrusively as to be imperceptible to the normal test viewer. If the substitution were distinguishable, it could, at least subconsciously, influence the response of the test viewer to the commercial. That is, were the viewer to know or suspect he was receiving a test commercial, he might react in a manner in which he believes he is expected to react, rather than acting normally, skewing the test results from his normal response. Therefore, it is desirable that the tuning be accomplished extremely rapidly so as to be indistinguishable. More specifically, the transition time between channels should be kept within about 60 microseconds to prevent an audible pop due to loss of the television signal intercarrier frequency modulated with the sound subcarrier. The normal and substitute channel tuning should be very accurately matched to ensure no shift in picture quality, particularly that of the chroma signal. The transition should be timed to occur during the vertical blanking interval between picture fields so that the change is not seen by the viewer.
Switching channels may require a large frequency change in the tuner. For example, if the normal channel is a low VHF channel (wherein Channel 2 has a picture carrier frequency of 55.25 MHz) and the substitute channel is a high UHF channel (wherein Channel 70 has a video carrier frequency of 807.25 MHz), the tuner might have to slew through more than 700 MHz. The vertical blanking interval of standard NTSC video, during which the substitution is to be effected, takes 1.3 milliseconds. The factor that is most critical in making the substitution indistinguishable is the sound. The audio stage of the television receiver is not tuned to the sound carrier, but is tuned to the 4.5 MHz intercarrier beat frequency generated between the video carrier and the sound carrier in each VHF and UHF channel. When the tuner of the receiver tunes between channels the intercarrier beat frequency disappears because both the video and sound carriers are no longer simultaneously present in the IF pass band. When the audio stage of the television receiver has no signal applied, its internal limiter amplifier will amplify noise up to an audible amplitude level. This causes the pop heard during viewer controlled channel changing. This presents no problem when the viewer changes channels, for it is to be expected. However, if an audible pop were produced during signal substitution, it would alert the viewer to the fact of substitution.
In order to avoid the effect of noise during signal substitution, the channel change must be sufficiently fast that the human ear cannot distinguish it. The total energy of the noise burst is the integral of power over time, but the human ear is essentially logarithmic in perception and can hear extremely low energy noise pulses. To make the noise attendant a channel change unobtrusive, the change should be accomplished in less than about 60 microseconds. Not only is extremely fast tuning required, but also the tuning must be relatively accurate to recover the 4.5 MHz intercarrier beat frequency. Due to the close proximity of the sound carrier of an adjacent channel to the video carrier of a substitute channel, a maximum error of about +500 KHz is required for both the picture and sound subcarriers of the substitute channel to be within the pass band.
Previous signal substitution systems have employed a cable television distribution system with a control box for channel switching located at each test viewer's home. These systems have employed a fast electronic tuner having a voltage controlled oscillator whose output frequency determined the channel to which the tuner was tuned. A voltage divider network established predicted tuning voltages necessary to cause the local oscillator to translate each individual channel's frequency to that of at least one channel of the television receiver. The tuner was made to select a particular channel very quickly by jamming the appropriate control voltage into the local oscillator, causing it to slew rapidly to the new frequency. This is known as jam tuning. Thus, by directing an electronic switch in the local oscillator control circuit to change from a normal channel voltage to the substitute channel voltage, a rapid substitution could be made. This prior art tuner controller system was predictive in nature in that the channel tuning control voltages corresponding to the desired input channels were determined by testing prior to or during installation of the control box at the home of the test viewer. A problem encountered was that with time the correct tuning voltages tended to drift.
Drifting resulted in frequency errors which caused loss of picture definition, and color hue and saturation changes. The automatic fine tuning circuitry in the television set of the test viewer might correct the error, but it would correct the error in a visible manner due to its slow operating speed. With time, the drifting became so extreme as to require that the control boxes be removed from test viewer homes for recalibration.
To extend the useful life of the control boxes without returning them to the shop for recalibration, a station-keeping feedback loop was added to the jam slewing. The electronic tuner assemblies for cable television signal substitution systems then employed a phase-locked loop feedback system which sampled the frequency output of the local oscillator in the tuner to determine if a frequency error were present. If such an error were present, the phase detector would provide an error signal for combination with the predicted voltage signal and application of a resultant voltage signal to the local oscillator of the tuner, thereby causing the tuner to provide the desired frequency output even after drifts such as caused by the aging of components. However, with time, due to aging of the components, the control values predicted for the various frequencies became more and more erroneous. This resulted in the voltage applied during the feed forward phase becoming so incorrect for the particular channel desired that the relatively slow operating phase-locked loop operated such that the viewer could perceive the substitution. Indeed, the initial voltage applied could become so erroneous as not to be able to tune to the proper channel. When the tuning became so impaired, the control box had to be returned to the shop for recalibration. Further, this type of control did not compensate for frequency errors in the received signals. Such errors are caused by transmission or conversion errors in the system ahead of the receiver.
Amplitude variation between the normal channel and the substitute channel also can make unobtrusive channel substitution difficult. The viewer can discern the substitution by a change in the visual quality of the picture. If the signal level changes too much, the television may fail to detect the synchronizing pulses and hence may fail to identify a video signal. The prior systems have taken no account of those problems.